Dendritic Growth tip velocities and radii of curvature in microgravity

Dendritic growth is the common mode of solidification encountered when metals and alloys freeze under low thermal gradients. The growth of dendrites in pure melts depends on the transport of latent heat from the moving crystal-melt interface and the influence of weaker effects like the interfacial energy. Experimental data for critical tests of dendritic growth theories remained limited because dendritic growth can be complicated by convection. The Isothermal Dendritic Growth Experiment (IDGE) was developed specifically to test dendritic growth theories by performing measurements with succinonitrile (SCN) in microgravity, thus eliminating buoyancy-induced convection. The first flight of the IDGE in 1994 operated for 9 days at a mean quasi-static acceleration of 0.7 × 10⁻⁶ g ₀. The velocity and radius data show that at supercoolings above approximately 0.4 K, dendritic growth in SCN under microgravity conditions is diffusion limited. By contrast, under terrestrial conditions, dendritic growth of SCN is dominated by convection for supercoolings below 1.7 K. The theoretical and experimental Peclet numbers exhibit modest disagreement, indicating that transport theories of dendritic solidification required some modification. Finally, the kinetic selection role for dendritic growth, VR ²=constant, where V is the velocity of the tip and R is the radius of curvature at the tip, appears to be independent of the gravity environment, with a slight dependence on the supercooling.     

Structural refinement of gray iron by electromagnetic vibrations

Simultaneous imposition of alternating electric and stationary magnetic fields on a molten metal will induce a vibrating motion in the liquid, which can lead to the formation and collapse of cavities and affect the solidification structure. Following earlier works on Al-Si alloys, the process is used to refine the microstructure of gray iron. It is found that in addition to the refinement of columnar-dendritic structure of primary austenite into a fine and homogeneous one, the eutectic cell structure is also extensively refined. The effects of the two main parameters involved in the process, that is, the frequency and the intensity of vibrations are, for the first time, quantitatively presented. The refinement of the cells proceeds as the frequency is increased up to about 500 Hz, where a reverse trend starts and results in a complete termination of the effects at about 10 kHz. The increase in the number of cells because of the increase in the intensity of vibrations shows a sharp jump at an electromagnetic pressure of about 10⁵ Pa, where the cavitation phenomenon is more likely to occur by overcoming the static pressure. However, increasing the electromagnetic pressure to higher values does not essentially result in a considerable further refinement, implicating the existence of a limit in the process of structural refinement by the cavitation phenomenon.     

Mathematical Modeling of the Solidification Structure Evolution in the Presence of Ultrasonic Stirring

Ultrasonic treatment (UST) was studied in this work to improve the quality of the cast ingots as well as to control the solidification structure evolution. Ultrasonically induced cavitation consists of the formation of small cavities (bubbles) in the molten metal followed by their growth, pulsation, and collapse. These cavities are created by the tensile stresses that are produced by acoustic waves in the rarefaction phase. The pressure for nucleation of the bubbles (e.g., cavitation threshold pressure) may decrease with increasing the amount of dissolved gases and especially with the amount of inclusions in the melt. Modeling and simulation of casting solidification of alloys with UST requires complex multiscale computations, from computational fluid dynamics (CFD) macroscopic modeling through mesoscopic to microscopic modeling, as well as strategies to link various length-scales emerged in modeling of microstructural evolution. The developed UST modeling approach is based on the numerical solution of the Lilley model (that is founded on Lighthills’s acoustic analogy), fluid flow, heat transfer equations, and mesoscopic modeling of the grain structure. The CFD analysis tool is capable of modeling acoustic streaming and ultrasonic cavitation. It is used in this work to study ingot solidification under the presence of ultrasound. The UST model was applied to low-temperature alloys including Al- and Mg-based alloys. Although the predicted ultrasonic cavitation region is relatively small, the acoustic streaming is strong and, thus, the created/survived bubbles/nuclei are transported into the bulk liquid quickly. The predicted grain size under UST condition is at least one order of magnitude lower than that without UST.     

Supercooling and Structure of Levitation Melted Fe-Ni Alloys

The effect of processing variables, such as supercooling, quenching rate, and/or growth inhibitors on the structure of levitation melted Fe-25 pct Ni alloys was investigated. The subgrain microstructure of samples solidified in water, molten lead, or ceramic mold was found to fall into three categories of dendritic, spherical, or mixed dendritic plus spherical morphologies. All three morphologies were observed when the samples had solidified with a superheat or a supercooling less than 175 K. At larger supercoolings, however, only the spherical morphology was observed. The structure fineness was shown to depend on the supercooling as well as on the solidification medium. The grain size and shape, on the other hand, was found to depend on the morphology of the microstructure, but not on the supercooling. The cooling rates during solidification and the heat transfer coefficient at the metal-quenching medium boundary were calculated. The coefficient for the samples solidified in water, molten lead, and ceramic mold was calculated to be 0.41, 0.52, and 0.15 w/cm² K, respectively.     

Effects of a High Magnetic Field on the Microstructure of Ni-Based Single-Crystal Superalloys During Directional Solidification

High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ′ phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/γ′ eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant (p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field.     

Data interpretation of “vapor pressure measurement of Zn-Fe intermetallic compounds”

It was with great interest that the article published by Mita et al. ^[1] was studied. In the article, experimental measurements of vapor pressures of various Zn-Fe intermetallic compounds were reported. Zinc activities were derived from the vapor pressure measurements. Such thermodynamic data, albeit indispensable for phase diagram construction, are scarce and often outdated as many researchers today opt to work on computation over experimentation for quick results. Data on zinc-related systems are even scarcer in comparison to other common metals such as iron, aluminum, or copper. It is certainly encouraging to see this experimental work on the Zn-Fe system.     

Room temperature compressive properties and strengthening mechanism of Mg_(96.17)Zn_(3.15)Y_(0.50)Zr_(0.18) alloy solidified under high pressure

The microstructures of Mg_(96.17)Zn_(3.15)Y_(0.50)Zr_(0.18) alloys solidified under 2-6 GPa high pressure were investigated by employing SEM(EDS) and TEM.The strengthening mechanism of experimental alloy solidified under high pressure is also discussed by analyzing the compressive properties and compression fracture morphology.The results show that the microstructure of experimental alloy becomes significantly fine-grained with increasing GPa level high pressure during solidification process,and the secondary dendrite arm spacing reduces from 40 μm at atmospheric pressure to 10 μm at 6 GPa pressure.The morphology of the second phases changes from the net structure by the lamellar-type eutectic structure at atmospheric pressure to discontinuous thin rods or particles at 6 GPa pressure.Besides,the solid solubility of Zn in the Mg matrix is improved with the increase of the solidification pressure.Compared with atmospheric-pressure solidification,high-pressure solidification can improve the strength of the experimental alloy.The compressive stre ngth is improved from 263 to 437 MPa at 6 GPa.The fracture mechanism of the experimental alloy changes from cleavage fracture at atmospheric pressure to quasi-cleavage fracture at high pressure.The main mechanism of the strength improvement of the experimental alloy includes the grain refinement strengthening caused by the refinement of the solidification microstructure,the second phase strengthening caused by the improvement of the morphology and distribution of the second phases,and solid solution strengthening caused by the increase of the solid solubility of Zn in the Mg matrix.     

Control of Superplastic Cavitation by Hydrostatic Pressure

It has been shown that the application of hydrostatic gas pressures during superplastic deformation of fine grained 7475 Al can entirely prevent the intergranular cavitation normally encountered at atmospheric pressure. A critical ratio of hydrostatic pressure to flow stress may be defined for each superplastic forming condition above which virtually no cavitation occurs. In superplastic deformation conditions where intergranular cavitation plays a significant part in final tensile rupture, the superplastic ductility may be improved by the application of hydrostatic pressures. Similarly, detrimental effects of large superplastic strains on service properties may be reduced or eliminated by the application of suitable hydrostatic pressures during superplastic forming. In this case, superplastically formed material may have the same design allowables as conventional 7475 Al sheet. Formerly with the Rockwell International Science Center, Thousand Oaks, CA.     

Experiments on the solidification structure of alloy castings

Two related experimental programs on the solidification structure of alloy castings are reported. In the first, the grain structure of catalytically clean Ni?Cu alloys is examined as a function of the degree of supercooling below the equilibrium liquidus. For supercoolings greater than 85°C, the Ni?Cu alloys exhibit a structure which is in accord with previous observations in pure nickel,i. e., the structure is found to be coarse and dendritic in the range 85° to 150°C supercooling, but fine and equiaxed for supercoolings greater than 150°C. However, in the lower range of supercooling (<85°C) the structure is fine and equiaxed. It is concluded that solute elements can promote grain formation in certain castings at supercoolings insufficient to cause heterogeneous nucleation. To study the effect of solute elements on the structure of a solidifying material, castings of pure nickel and aluminum are compared with binary alloys of these base materials. Over the composition ranges studied in both alloy systems, the structure is observed to be related to a parameter which includes the slope of the liquidus, the bulk solute concentration, and the solute distribution coefficient. It is shown or is argued that heterogeneous nucleants are not involved and therefore another mechanism must be operating in determining the structure. In any event, these latter experiments suggest that an effective means of controlling the structure of castings is by appropriate selection of alloying additions on the basis of the alloy variables contained in such a parameter.     

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed. Formerly with Massachusetts Institute of Technology, Cambridge, MA.     

Liquid separation effects in Fe−Cu alloys solidified under different cooling rates

The effect of cooling rates on the microstructure of Fe−Cu alloys was investigated. A variety of solidification techniques was employed, in order to obtain a wide range of cooling rates. At high cooling rates (about 10⁴ K/sec), and in the composition range 30 to 80 wt pct Cu, the microstructures showed clear evidence of metastable liquid separation. This indicates a melt supercooling of about 50 to 100 K. Liquid separation coupled with high interfacial velocities resulted in solute trapping, and in a spherical morphology for one of the solids. At cooling rates lower than 10⁴ K/sec no liquid separation was observed, and the alloys solidified in a conventional manner,i.e., with a polycrystalline or a dendritic microstructure, depending on the Cu content. The type of the γ-Fe to α-Fe solid state transformation, taking place during cooling after solidification, depends on the cooling rates as well as on the Cu content in the γ-Fe phase. At medium cooling rates the transformation is martensitic, while at low or high cooling rates a polycrystalline transformed structure is obtained. A. MUNITZ, formerly Visiting Research Associate at the University of Florida at Gainesville, FL 32611     

Banded solidification microstructures

Banded microstructures are composed of alternate structures or phases which develop mostly parallel to the transformation front. At low growth velocities, bands of the same microstructure but with different scales form through periodic fluctuations of the solidification system. On the other hand, banding can occur as a transformation microstructure when the growth front becomes unstable to oscillations. This instability is either due, at low velocities, to nucleation of another phase (peritectics) or, at high velocities, to nonequilibrium effects at the interface which lead to periodic changes of the microstructure. In this article, the inherent banded patterns of low velocity peritectic solidification and high velocity nonequilibrium solidification will be presented and their origin will be discussed. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

The distribution of silver,gold, platinum and palladium in metal-matte systems

The distribution of silver between molten metal and matte was determined at temperatures of 1150 to 1250°C. The partition coefficient (K_Ag-ratio of wt pct silver in matte to wt pct silver in metal) was 0.46 and was essentially independent of silver concentration, temperature, iron content and oxygen and SO₂ partial pressures over the ranges studied. Only the presence of nickel significantly affected K_Ag, causing it to rise from 0.46 to 1.16 within the region of liquid immiscibility. Outside this region all silver was intimately associated with chalcocite, rather than the heazel woodite or metallics, formed during solidification. Studies on other precious metals showed that K_Au=8×10^?3, K_Pd=6×10^?3 and K_Pt=4×10^?4 in the Cu?S system at 1200°C. Application of these results to actual operations is discussed, as is the chemical behavior of precious metals during matte smelting.     

Thermodynamics of lead and bismuth in CaO–CaF2 melts

In order to understand a new refining process for removing tramp elements from alloy steels under strongly reducing conditions, the authors previously reported the thermodynamic behaviour of P, Sn, Sb, As and Cu in CaO–CaF₂ melts. In this investigation, the thermodynamic behaviour of Pb and Bi in CaO–CaF₂ melts under oxygen partial pressures from 10^?16 to 10^?20 atm was studied at temperatures ranging from 1360 to 1550°C, using a similar technique. It was shown that Pb and Bi exist in slag mainly as Ca₂Pb and Ca_1.5Bi, respectively, under very low oxygen partial pressure although a part of Pb is soluble in slags as metal. The obtained results were extrapolated to lower oxygen partial pressures to investigate the possibility of removing these tramp elements from molten iron, causing Bi to be removed from molten iron below an oxygen partial pressure of 10^?23 atm, whereas Pb is removed under higher oxygen partial pressures.     

Blowhole formation during solidification of liquid steel

This paper deals with an analysis of the conditions leading to the formation of blowholes during the solidification of liquid steel. The peritectic reaction, the effects of solidification on surface tension, the enrichment of solutes and reactions between them during the solidification are calculated and discussed. The results show that the main gases contributing to blowhole formation in liquid steel are H₂ and CO, the respective equilibrium partial pressures and their sum of which exhibit maxima at the start of peritectic reaction of liquid steel during the solidification. When this maximum pressure exceeds the sum of atmospheric and ferrostatic pressures and additional pressure from surface tension, the blowholes would be formed. By this assumption the critical equation and domain diagram for controlling blowhole formation have been obtained.     

Effects of the intensity and frequency of electromagnetic vibrations on the microstructural refinement of hypoeutectic Al-Si alloys

An experimental apparatus that uses a superconducting magnet and enables the simultaneous application of an alternating electric field with a frequency of up to 50 kHz and a magnetic field of up to 10 T was designed and assembled. Electromagnetic vibrations were induced in Al-7 wt pct Si alloy during solidification by simultaneous application of the two fields. The thorough investigation, which was carried out over wide ranges of intensity (an electromagnetic pressure range of 0 to 2.25×10⁵ Pa) and frequency (0 to 50 kHz), clarified the effects of the two main parameters on the microstructural refinement brought about by electromagnetic vibrations. Low-intensity vibrations changed the highly columnar dendritic structure into one composed of large, equiaxed dendrites. As the intensity, and consequently, the magnetic pressure were increased, at about 0.93×10⁵ Pa, fine isolated grains started to appear and dominated the structure as the pressure was increased further. At low frequencies, the structure was one with large, equiaxed dendrites, which disintegrated to form a fine and homogeneous structure as the frequency was increased. At about 1.5 kHz, the trend reversed and the structure gradually became a completely columnar dendritic one at frequencies higher than 10 kHz. Metallographic observations showed that the cavitation phenomenon has been a main factor behind the observed microstructural refinement. The effects of mechanical vibrations of the experimental apparatus were also investigated and found to have no contribution to the observed effects.     

Nanocomposites of Aluminum Alloys by Rapid Solidification Processing

Aluminium alloys reinforced with transition metal aluminide (Al₃Ti, Al₃Fe, Al₃Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al?CTi and Al?CFe alloys to produce nanocomposites.     

Simulation of Microsegregation in Multicomponent Alloys During Solidification

A phase‐field model is applied to the simulation of microsegregation and microstructure formation during the solidification of multicomponent alloys. The results of the one‐dimensional numerical simulations show good agreement with those from the Clyne–Kurz equation. Phase‐field simulations of non‐isothermal dendrite growth are examined. Two‐dimensional computation results exhibit different dendrites in multicomponent alloys for different solute concentrations. Changes in carbon concentration appear to affect dendrite morphology. This is due to a larger concentration and a lower equilibrium partition coefficient for carbon. On the other hand, changes in phosphorus concentration affect the dendrites and interface velocity in multicomponent alloys during solidification when phosphorus content is increased from 10^?3 mol% P. With additional manganese, the solidification kinetics slow down; dendrite morphology, however, is not affected. The potential of the phase‐field model for applications pertaining to solidification has been demonstrated through the simulations herein.